1. Field of the Invention
The present invention relates to a novel process for preparing substituted biphenyls of the formula (III) by reacting arenes of the formula (I) with arenes of the formula (II) in the presence of a transition metal catalyst and of at least one oxidizing agent.

2. Description of Related Art
Biaryl compounds, especially biphenyl compounds, are of industrial significance as fine chemical is intermediates for pharmaceuticals, optical brighteners and agrochemicals.
It is known that substituted biphenyls are obtainable by Suzuki couplings. For this purpose, however, costly borinic/boronic acids with haloarenes are required as starting compounds, these being coupled to one another in the presence of a transition metal catalyst; cf. WO 2011/023324 A1. Recently, the synthesis of biphenyls by direct double C—H activation has been developed as an attractive alternative to the existing synthesis methods (e.g. Suzuki reaction). In this method, arenes bearing a directing group are reacted with arenes in the presence of transition metal catalysts and/or suitable activating reagents. Review articles on this topic can be found, for example, in Charles S. Yeung, Vy M. Dong, Chem. Rev. 2011, 111, 1215-1292. A review of Pd-catalysed aryl-aryl coupling by double C—H activation is given by Shu-Li You and Ji-Bao Xia, Top. Curr. Chem. 2010, 292, 165-194. A review of oxidative cross-couplings of arenes can be found in James A. Ashenhurst, Chem. Soc. Rev. 2010, 39, 540-548. The particular advantage of direct double C—H activation is the enhanced sustainability compared to the existing synthesis methods (e.g. Suzuki reaction). No pre-functionalized coupling partners are required, as a result of which the number of reaction steps and waste are reduced.
However, in this reaction type, there are also a multitude of difficulties, for example unfavourable thermodynamics, the generally low reactivity of C—H bonds and selectivity problems (functionalization of a C—H bond in the presence of other C—H bonds, and the competition between hetero and homo coupling). In the Pd-catalysed aryl-aryl couplings through double C—H activation described in the prior art, the descriptions by, for example, Gordon Brasche, Jorge García-Fortanet, Stephen L. Buchwald in Org. Lett. 2008, 10(11), 2207-2210 are restricted to the reaction of anilides with electron-rich arenes (containing electron-donating substituents such as Me, OMe, etc.). As well as the palladium catalyst, DMSO in combination with pure oxygen, and also TFA in combination with Na2S2O8, are used here as additional activating reagents or oxidizing agents. Electron-poor aromatics (containing electron-withdrawing substituents such as F, CF3, CHF2, Cl, etc.) are considered too unreactive and can therefore be reacted with anilides under these reaction conditions only with very great difficulty, if at all.
Bi-Jie Li, Shi-Liang Tian, Zhao Fang, and Zhang-lie Shi, in Angew. Chem., 2008, 47, 1115-1118, disclose processes with multiple C—H activation for preparation of biologically active molecules, the process being free of organohalogen and organometallic compounds. For example, a process for palladium-catalysed ortho-arylation of acetanilide with ortho-xylene using O2 as an oxidizing agent and PrOH as a solvent is disclosed (Tab. 1, No. 6). Also disclosed is the coupling of an electron-poor arene (fluorobenzene) with N-acetyltetrahydroquinoline (Tab. 2, No. 8). However, a significantly higher amount of catalyst was required for this, the reaction was not stereoselective and the yield was only 48%.
Joanna Wencel-Delord, Corinna Nimphius, Frederic W. Patureau and Frank Glorius describe, in Angew. Chem. Int. Ed. 2012, 51, 2247-2251, an Rh-catalysed dehydrogenating aryl-aryl coupling which allows the use of electron-poor aromatics. To date, however, these conversions by C—H activation have been performed only with benzamides as directing coupling partners. The additives or oxidizing agents used here have been PivOH and CsOPiv, and also AgSbF6 and Cu(OAc)2. The use of anilides in the reaction with electron-poor arenes (containing electron-withdrawing substituents, for example F, CF3, CHF2, Cl, etc) has not been possible to date in the processes previously described in the prior art.